This invention relates to a method of recording high contrast (gradient) and density images reproduced by electrophotography.
Conventional radiography intended for use in medical diagnoses is performed by either the X-ray photographic process in which a radiation image is directly exposed on an X-ray film or by a radiation image information record/reproduce system in which an image exposed on a stimulable phosphor sheet is read photoelectrically and appropriately processed to provide a reproduction on a CRT, the X-ray film or a photographic film. Both approaches have achieved marked results. In other systems such as CT and MR which produce images for use in medical fields, images for use in medical diagnosis are reproduced on photographic films to provide hard copies. Other images in hard copy that are currently employed for diagnosis in research or engineering fields include the electron microscope image produced from an electron microscopic image record/reproduce system using a stimulable phosphor sheet, as well as a probe image from a radiation probe system employed in the nondestructive inspection of buildings and other structures.
Such images for use in diagnoses, particularly in medical diagnoses, are required to have high image quality, namely, high density, high resolution and high contrast or multiple gradations. This need has heretofore been met by a wet system, in which an exposed silver salt photographic material is wet processed to provide image reproduction. The wet system is capable of reproducing a high-quality image but, on the other hand, it is not simple to use because of the need to employ processing solutions such as developing and fixing baths. Under the circumstances, a growing interest is emerging in the field of image recording by a dry system which involves no wet processing.
A commercially feasible dry system for image recording is by electrophotography, which comprises the basic steps of imagewise exposure of an electrophotographic photoreceptor to form a latent image, developing the latent image with a toner to form a visible image and transferring the toner image on to a final image support such as a paper or a film, thereby obtaining the transferred toner image in hard copy.
In electrophotography, the reproduced monochromatic image is formed of the toner particles transferred on to the support and its contrast is represented by two areas of the support, one to which the toner has been transferred and the other where it has not been transferred. Hence, the degree of contrast that can be achieved in the recording of a reproduced image by one cycle of exposure, development and transfer steps using a monochromatic toner is limited and irrespective of whether the support is of a reflection type such as paper or of a transmission type such as a film, the degree of contrast that is required by images for use in diagnoses, particularly in medical diagnoses, which should be at least comparable to 10 bits (1024 levels of the gradient) cannot be attained.
A further problem with such one-pass image recording involving only one cycle of exposure, development and transfer steps is a limited amount of transfer that can be effected at a time and the attainable maximal density is lowered, even becoming insufficient when using a transmission support. In order for a given image to have sufficiently high quality that it is suitable for use in medical diagnosis, a maximal density of about 3.0 is necessary but this need can hardly be met by one-pass image recording and the quality of the resulting image is far from being satisfactory.